1. Field of the Invention
The invention relates to an improved process for the hydrogenation of coal. More particularly, the invention provides a technique for the utilization of waste water generated during conventional coal hydrogenation processes.
2. Description of Prior Art
Coal hydrogenation is normally understood to mean the addition of hydrogen to coal under pressure and under cracking conditions in the course of which catalysts are employed. During the hydrogenation process, oxygen contained in the coal is partly converted into tar acids, that is, into phenols, cresols and xylenols. A further portion of the oxygen leads to the formation of reaction water during the hydrogenation process. Following the hydrogenation reactor, there are precipitators in which the hydrogenated products are separated. The separation of a coal-oil phase and aqueous phase is effected in the so-called cold precipitators.
Quenching water is injected into the cold precipitator in order to prevent blockages due to salt deposition in the pipe lines of the hydrogenation plant subsequent to the cold precipitators. This quenching water, together with reaction water, forms an aqueous phase in the cold precipitator. Tar acids present in the coal are highly water soluble products. As a result, tar acids become a significant constituent of the reaction water and quenching water products.
Additionally, further quantities of tar acid containing waste waters are formed during the atmospheric distillation of the coal oil obtained in the hydrogenation operation, which is carried out with the addition of hydrogen. Tar acid containing waste water is also found in the condensate of hydrogenation residue from the vacuum distiller's steam input.
Because of the high tar acid content, as well as the hydrogen sulfide and ammonium which waste waters from a hydrogenation plant contain, these waste waters cannot be discharged into the open waters for obvious environmental considerations. It is the conventional practice to feed these contaminated waste waters to a phenol extracting plant and a subsequent following waste water treatment system after having separated out the hydrogen sulfide and ammonium. The waste water, thus treated, is generally suitable for discharge. This conventional treatment requires a considerable expenditure in both equipment and energy which becomes economically undesirable the greater the amount of waste water to be treated.
One such proposal to resolve the aforementioned waste water problem can be found in German Patent Publication No. 30 36 259 A 1 which seeks to employ the tar acid containing waters in a gasification plant, which is connected with a hydrogenation plant. This plant serves to produce hydrogen from a vacuum residue or coal. The process is such that the vacuum residue or the coal is introduced as a water suspension into a reactor. The suspension consists of water which contains tar acids. This possible solution to the aforedescribed waste treatment process does not require the interposition of purification stages for the waters. However, where no such gasification is available, the use of the aforedescribed waste treatment process is still required.